Lead Scientist Abstract

Southern Africa is the world's largest emitter of biomass-burning aerosols. Their westward transport over the remote southeast Atlantic Ocean colocates some of the largest atmospheric loadings of absorbing aerosol with the least examined of the Earth’s major subtropical stratocumulus decks. When the light-absorbing smoke is above a bright surface such as a low cloud deck, the net atmospheric absorption of sunlight increases, compared to a smoke-free environment. This constitutes a warming impact on climate. Because of the smoke, global aerosol models highlight that the largest positive direct radiative effect in the world occurs in the southeast Atlantic. Yet, this region also exhibits large differences in magnitude and sign between reputable models, partly because of differences in representing the underlying cloud distributions. In addition, some observations suggest positive direct radiative effects far exceeding those of global aerosol models. Many uncertainties contribute to the highly variable model radiation fields for this region: the aging of the shortwave-absorbing aerosol during transport and how this affects the aerosol radiative properties, how much of the aerosol mixes into the cloudy boundary layer, and how the low clouds adjust to smoke-radiation and smoke-cloud interactions. LASIC (Layered Atlantic Smoke Interactions with Clouds) is a field campaign to improve understanding of aged carbonaceous aerosol and the mechanisms by which clouds adjust to the presence of the aerosol. The first ARM Mobile Facility's (AMF1) cloud, aerosol, and atmospheric profiling instrumentation is deployed to Ascension Island (8 degrees south latitude and 15 degrees west longitude), located within the trade-wind shallow cumulus regime 3,000 kilometers offshore continental Africa. This is within the latitude zone of the maximum outflow of aerosol, with the deepening boundary layer known to entrain free tropospheric smoke. The primary activities for LASIC are to:

• improve current knowledge on aged biomass-burning aerosol and its radiative properties
• use surface-based remote sensing to sensitively examine the atmosphere for the relative vertical location of aerosol and clouds
• improve understanding of the cloud adjustments to the presence of shortwave-absorbing aerosol within the vertical column, both through aerosol-radiation and aerosol-cloud interactions.

The measurements will span June 1, 2016, to October 31, 2017, which encompasses two biomass-burning seasons. LASIC contributes to an international, coordinated strategy focused on the southeast Atlantic. It will overlap with the National Aeronautics and Space Administration (NASA) ORACLES-2016 and U.K. CLARIFY aircraft campaigns from August to September 2016, and the NASA ORACLES-2017 and National Science Foundation ONFIRE aircraft campaigns in 2017 (see Related Campaigns link). St. Helena Island (15 degrees south latitude and 5 degrees west longitude), located upwind of Ascension within the boundary layer flow and directly west of the main stratocumulus deck, will receive modest secondary instrumentation as part of CLARIFY, LASIC, and NASA. Smoke and clouds over the remote ocean represent a regime of significant climatic importance that has not yet been examined with comprehensive surface-based measurements. Ascension Island is strategically located to collect observations with which to resolve current uncertainties in the aging and transport of smoke and its interactions with low clouds. These processes control the spatial and vertical distribution of the Earth's radiative balance at a location with important cloud feedbacks to climate. The long-term, high-time-resolution measurements from an AMF1 deployment provide a stringent test for global aerosol models.